Of recent years there has been a trend toward operating spark-ignited internal combustion engines at leaner air/fuel ratios in order to decrease the amount of hydrocarbon and carbon monoxide in the exhaust. One limiting factor in going towards leaner operation is gasoline maldistribution from cylinder-to-cylinder. The inducted air/fuel ratio must be kept lean enough such that the cylinder supplied with the leanest mixture will still fire, otherwise exhaust hydrocarbon and carbon monoxide emission will increase rather than decrease. The problem is most acute when the choke is first moved to the open or off position because, at that time, the carburetor and associated hardware are not up to operating temperature and much of the inducted gasoline remains in liquid form causing excessive maldistribution.
Several methods have been proposed to minimize the problem. According to one method the entire intake manifold on which is mounted the primary venturi carburetor is heated by providing an exhaust gas jacket around the intake manifold (Bartholomew, "Potentialities of Emission Reduction by Design of Induction Systems," S.A.E. Meeting January 1966, Detroit, Michigan). According to another method a "hot spot" is provided in the intake manifold directly below the primary venturi. This is accomplished by having a thin sheet metal plate separate the exhaust cross-over from the intake manifold at this location (W. D. Bond, "Quick-Heat Intake Manifolds for Reducing Cold Engine Emissions," S.A.E. Meeting October 1972, Tulsa, Oklahoma). Both of these methods are effective but do not eliminate the problem.
Other related induction systems resulting from a prior art search and listed in chronological order are:
Kambak, U.S. Pat. No. 1,479,547 (1924) which discloses an induction system in which liquid fuel is delivered into a bulbous chamber located in the exhaust manifold. The liquid fuel is vaporized and then mixed with air which is inducted through separate means.
Maroger, Fr. Pat. No. 629,582 (1926) which discloses an exhaust-jacketed heat exchanger placed between the carburetor and the engine intake.
Mock, U.S. Pat. No. 1,777,472 (1930) which employs a thin wall "hot spot" in the carburetor adjacent to the power jet to assist in vaporization of excess gasoline inducted during acceleration.
Duthoit, U.S. Pat. No. 2,066,720 (1937) which discloses an updraft carburetor in which the air/fuel mixture is conducted through a bulbous conduit which extends transversely through the exhaust manifold.
Titus, U.S. Pat. No. 2,720,197 (1955) which discloses a liquid-heated heat exchanger installed between the carburetor and the intake manifold.
Summers, U.S. Pat. No. 3,016,051 (1962) disclosing a two-barrel induction system for a V-type engine which includes a U-tube which connects the two separate branches of the intake manifold. The U-type extends into the exhaust cross-over.